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                [Hnoi2013]游走                </h1>
                <p>时间限制：10s&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;  空间限制：128MB</p>			</div>
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                                <h3>题目描述</h3><p><p><span style="font-size: medium">一个无向连通图，顶点从1编号到N，边从1编号到M。 <br />
小Z在该图上进行随机游走，初始时小Z在1号顶点，每一步小Z以相等的概率随机选 择当前顶点的某条边，沿着这条边走到下一个顶点，获得等于这条边的编号的分数。当小Z 到达N号顶点时游走结束，总分为所有获得的分数之和。 <br />
现在，请你对这M条边进行编号，使得小Z获得的总分的期望值最小。 </span></p></p><hr/><h3>输入格式</h3><p><p><font size="4">第一行是正整数N和M，分别表示该图的顶点数 和边数，接下来M行每行是整数u，v(1&le;u,v&le;N)，表示顶点u与顶点v之间存在一条边。 输入保证30%的数据满足N&le;10，100%的数据满足2&le;N&le;500且是一个无向简单连通图。 </font></p></p><hr/><h3>输出格式</h3><p><p><font size="4">仅包含一个实数，表示最小的期望值，保留3位小数。 </font></p></p><hr/><h3>样例输入</h3><pre>3  3                
2  3
1  2
1  3
</pre><hr/><h3>样例输出</h3><pre>3.333</pre><hr/><h3>提示</h3><p><div style="margin: 0.25pt -1.5pt 0pt 39pt; line-height: 12.75pt" align="left"><span style="font-size: 11pt; color: black">边</span><span style="font-size: 11pt; color: black">(1,2)</span><span style="font-size: 11pt; color: black">编号为</span><span style="font-size: 11pt; color: black">1</span><span style="font-size: 11pt; color: black">，边</span><span style="font-size: 11pt; color: black">(1,3)</span><span style="font-size: 11pt; color: black">编号</span><span style="font-size: 11pt; color: black">2</span><span style="font-size: 11pt; color: black">，边</span><span style="font-size: 11pt; color: black">(2,3)</span><span style="font-size: 11pt; color: black">编号为</span><span style="font-size: 11pt; color: black">3</span><span style="font-size: 11pt; color: black">。</span></div></p><hr/><h3>题目来源</h3><p>非官方数据</p>
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